An embodiment of the present invention is a radiation emission device, in particular, a rotating-anode X-ray tube with improved bearing and a method for its manufacture. An embodiment of the present invention can be used in medical imaging and also in the field of non-destructive controls when high-powered X-ray tubes are used.
In radiology, such X-rays are produced by an electron tube provided with an anode rotating on a shaft. A powerful electrical field created between the cathode and the anode causes electrons emitted by the cathode to strike the anode, generating X-rays. For this X-ray emission, the positive polarity is applied to the anode by its shaft, and the negative polarity is applied to the cathode. The unit is insulated, especially by dielectric pieces or by an enclosure of the electron tube. This enclosure can be partly made of glass.
When the tube is high-powered, the impact of the electrons on the anode has the effect of abnormally heating this anode. If the power is excessively high, the emitter track of the anode may get deteriorated and pitted with impact holes. To prevent such overheating, the anode is made to rotate so that a constantly renewed and constantly cold surface is presented to the electron stream.
A motor of the tube therefore drives the shaft of the anode freely in a mechanical bearing. This shaft is located in an anode chamber. The anode chamber is itself formed in a support of the anode. On the one hand, the bearing is held by the anode support and, on the other hand, it holds the shaft of the anode.
In practice and when made on an industrial scale, this bearing has classic ball bearings as opposed to the little-used magnetic bearings. The problem posed by the rotating anode arises from the fast wearing out of the metal coating the balls during the rotation of the shaft in the bearing. The service lifetime is then about 100 hours, giving a period of use of the tube of about six months to one year. To overcome this problem, it has been proposed to coat the balls of the bearing with metal, lead or silver in the form of a thin layer. To reduce this premature wearing out of the metal layer, in the known art, a lubricant film is placed at the interface between the surfaces of the ball bearings and the shaft, between the bearing and the shaft of the anode. To this end, the interior of the chamber is filled with a gallium-indium-tin based liquid. Such a liquid is chosen because it improves the coefficient of friction, reduces the noise of the impacts between the balls and augments heat transfer through the heating of the shaft from the anode to the fixed part, either by convection or by conduction. Other lubricant liquids are not chosen because they have poor degassing properties.
The use of the gallium-indium-tin based alloy has proved to be a source of difficulty. Indeed, this alloy, which is liquid at ambient temperature (starting from 10 degrees Celsius), gets oxidized very quickly in contact with air. This oxide is solid and takes the form of a surface film within a very short time of about one to two minutes. This means that the handling of such a liquid in industrial conditions has to be done with certain precautions, in a neutral atmosphere or under vacuum. Besides, this film has no lubricating quality, indeed far from it. Gallium is furthermore highly corrosive. If this mixture were to be handled, even in the laboratory, liquid could get spilt or could leak or overflow, giving rise to puddles or deposits on the handling surface. It is then extremely difficult to remove all these puddles or deposits in a white room, especially in a system under manufacture (in the enclosure of the tube). Indeed, if a stain is wiped off, it reappears within a few seconds in the form of another brownish stain at the position that has been just (but not completely) cleaned. The state of the room is then not propitious to the requirements of quality manufacturing.
The difficulties then are of two types: the handling of the alloy itself in the laboratory or plant, and its mode of insertion under vacuum into the bearing during the manufacture of the tube. Furthermore, the purity of this liquid, despite its contribution to the lubrication of the bearing in association with the balls of the bearing, may deteriorate in the course of time and finally, as in the case of the coating of the balls, it may cease to have any effect.
In current and future radiology, the power need by electron tubes are increasing in order obtain improved diagnosis. This increase in power is increasing the weight of the anode to six-eight kilograms. The resulting effects within the bearing are becoming critical. Furthermore, for use in computerized tomography with continuous rotation at two rotations per second, the bearing is subjected to acceleration of about eight G. Rotation speeds of three to four rotations per second are anticipated. As a consequence, the service life of the bearing, and therefore of the tube, with the balls and the liquid, may be limited in time. Indeed, the liquid may lose its properties and therefore its qualities as and when heating and friction occur inside the bearing.